TH 2.5 is a B cell hybrid which can be induced to secrete IgM by stimulation with LPS and anti IgM in the absence of lymphokines. Cells from this line were stimulated with LPS. Stimulated and unstimulated cells were subjected to limiting dilution analysis for their ability to form large clones. Induction with LPS reduced cloning efficiency at least 50-fold. Most cells still produced microscopic clusters which did not continue to grow. Those clones which grew vigorously in these conditions were subcultured and reanalyzed. No selection for resistance to LPS was observed; induced cells again showed a greatly reduced cloning efficiency when exposed to LPS. The results suggest that the induction of immunoglobulin secretion with LPS commits many of these cells to terminal differentiation. An as yet undetermined factor(s) seems responsible for protecting some cells from this commitment, but clones derived from such "protected" cells are phenotypically similar to or identical to the original hybrid. We have obtained four B cell hybrids which respond to BSF-1. Two show an increased rate of growth, but no change in membrane Ia. Two others show increased expression of Ia antigens on the membrane, but no change in rate of growth.